In an ink jet system printer, an ink stream emitted from a nozzle breaks into drops due to the Rayleigh's instability phenomenon. Although an exciting frequency of ultra-sonic vibration applied to the nozzle, and hence, to the stream of ink drops, is substantially identical with the drop separation frequency, the phase of drop separation or formation varies unsteadily. These variations will be caused by the blocking up of the nozzle due to foreign substances, temperature dependent characteristics of viscosity and surface tension of the ink and so forth, and in fact will be unavoidable. In an ink jet system printer of the charge amplitude controlling type wherein the ink drops charged with the charging signals are electrostatically deflected in accordance with the charge amplitude thereon as they pass through a high-voltage electric field thereby printing desired symbols such as alphabet characters, it is of importance that the application of the charging signals or the phase of the charging signals is timed to be in agreement with the drop separation phase. Such phase synchronization will exert a strong influence upon character formation and printing quality.
In the past, one approach to the phase synchronization has been proposed, wherein, when a printing head carrying the nozzle, charging electrode, etc., is returned to the left end or home position, the exciting signals for the ultra-sonic vibration and charging signals are both present in the optimum phase relation. However, this cannot follow variations in the drop separation phase which occur during a period of one-row printing, i.e., the period in which the printing head traverses the width of the record medium on which the printing is effected.